- Bragg Gratings Enable Efficient Phosphate Glass Fiber Lasers
Phosphate fibers have advantages over conventional silica fibers.
Although silica glass is the material of choice for nearly all of today’s fiber lasers, the rare-earth doping level that can be achieved in silica glass is a significant limitation to its performance in lasers, especially in pulsed lasers. Fibers made from phosphate glass can accommodate higher doping levels and, therefore, might be more appropriate for many laser applications. However, they have one critical drawback: Until now, engineers have been unable to write reflective-index gratings into phosphate glass fibers.
Thus, phosphate glass fiber lasers have been forced to use spliced-on silica glass fiber Bragg gratings or thin-film coatings as resonator mirrors. Both work-around solutions introduce troubling mechanical and optical problems to the laser.
Recently, however, researchers at Carleton University in Ottawa and at the University of Arizona in Tucson successfully wrote effective fiber Bragg gratings in a single-mode, phosphate glass fiber that they constructed using glass provided by NP Photonics Inc. of Tucson. They wrote the gratings in holey fibers and in rare-earth-doped fibers, and they believe that their success will lead to short monolithic phosphate glass fiber lasers with high power, good spectral quality and low fabrication costs.
Figure 1. The researchers launched amplified spontaneous emission from an erbium-doped fiber amplifier into the phosphate glass fiber as they wrote the grating and monitored its real-time reflectivity. The shift of the center wavelength of the reflected spectrum indicates a change in the effective refractive index of the fiber (because the center wavelength is given by 2neff Λ, where Λ is the period of the fiber Bragg grating). Images reprinted with permission of Applied Physics Letters.
An ArF laser from GSI Lumonics in Novi, Mich., provided the 193-nm radiation for writing the fiber Bragg gratings. The 14-ns pulses contained 80 mJ and provided a fluence of 400 mJ/cm2 at the fiber. A phase mask with a 976.3-nm period in front of the fiber produced a fringe pattern of 488.15 nm on the fiber. The contrast of the fringe pattern was not the most satisfactory because the mask was optimized for 248 nm rather than 193 nm. Nonetheless, the gratings had excellent spectral characteristics.
The researchers spliced a standard telecom fiber to their phosphate fibers and monitored the reflectivity of the gratings as they were written (Figure 1). After ~10 minutes of exposure to the ultraviolet radiation, the fiber Bragg gratings’ reflectivity exceeded 95 percent, and the refractive-index modulation amplitude was ~10–4.
Figure 2. The fiber Bragg gratings’ (FBG) refractive-index modulation depth increased and eventually stabilized with hundreds of hours in an oven. This improvement of performance is the opposite of the effect usually observed in optically written gratings and bolsters the scientists’ belief that the gratings may be useful in practical lasers.
Thermal degradation is a problem with many optically written gratings, but the refractive-index modulation of these phosphate glass gratings increased rather than decreased with hundreds of hours in an oven at 100 °C and higher (Figure 2). Even a minute-long, 400 °C blast from a heat gun merely reduced the fiber Bragg gratings’ reflectivity by 0.2 percent. Reflectivities above 99 percent have been achieved with a 14-mm-long grating.
Figure 3. The reflectivity spectrum of the fiber Bragg grating indicates that it would be well suited for erbium-doped fiber lasers in the telecom C-band.
The reflectivity spectrum of the gratings was very clean, with a full width at half maximum of 0.14 nm (Figure 3). The absence of any spectral features on the low-wavelength side of the reflectivity peak indicates that the refractive-index modulations are well aligned and perpendicular to the fiber axis, and that the fringe pattern is uniform across the fiber’s cross section.
Applied Physics Letters, Sept. 4, 2006, 101127.
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